Answer to Question #13693 Submitted to "Ask the Experts"

What is the average radiation dose for a whole-body dual energy x-ray absorptiometry (DEXA) bone scan? I looked at an x-ray risk website that only gave data for a "standard exam." I don't think that option applies to the whole body.

Your question about x-ray doses in bone densitometry is a good one. These exams are very common and questions about the x-ray doses associated with them are often deflected because the dose is so low. It is true that these scans, technically known as DEXA, result in some of the lowest effective doses to patients that occur in x-ray exams used in medicine. However, there is no reason not to publicize exactly how much that is. The main reason that the dose from DEXA can be so low compared to other x rays of the skeleton is that this exam doesn't use enough radiation to make a high-quality image of bones. It is performed with just enough radiation to sense the difference in transmission through bone of two different energies of x rays. This measurement allows the determination of bone density and calcium content, which are both indirect measurements of the risk of fractures due to osteoporosis.

DEXA are sometimes performed over just the lumbar spine and hips because the density in these regions have been shown to correlate well with diagnosis of osteoporosis. The scans can be done over the entire body to obtain more comprehensive data.

As you probably know, there are a number of different ways to specify radiation dose. The most widely used quantity is the Sievert (Sv). It takes into account the energy deposited by radiation as well as the part of the body irradiated to arrive at a unit that reflects overall potential for harm to cells in the body. For radiation therapy we want to harm tumor cells and we use doses of the order of several Sv. In diagnostic radiology we use the smallest doses that allow us to make accurate images of the body with x rays. This requires much less radiation, in the range of thousandths of sieverts or millisieverts (mSv)

Here are some effective doses associated with DEXA and a few other exams in diagnostic radiology for comparison:

• DEXA—Lumbar spine and hip 0.001 mSv
• DEXA—Whole body 0.003 mSv
• Extremity (hand, foot, etc.) x ray—0.001 mSv
• Dental X Ray—0.005 mSv
• Chest X Ray—0.1 mSv
• Spine Computed Tomography (CT) Scan—6 mSv
• Abdomen and Pelvis CT—10 mSv

We usually compare radiation doses with "natural background," the radiation we receive by living on Earth, through radioactive isotopes all around us in soil, air, building materials, etc. We also receive some radiation from cosmic ray sources that exist in the universe, including the star closest to us, the sun. The average annual natural background radiation level in the United States is approximately 3 mSv. No study has ever shown that radiation from natural background sources over an entire lifetime is associated with higher risks of disease or death, anywhere on Earth even though actual background levels in some regions of the planet are five to ten times that of the United States. It is assumed that any amount of radiation is associated with some risk, but levels associated with many years of natural background seem to have no discernable health effects.   

So, in terms of natural background in the United States, an abdomen and pelvis CT is about three years' worth of natural background, while the whole-body DEXA is equivalent to about nine hours.  

E. Russell Ritenour, PhD, FAAPM, FACR, DABR